1. Field of the Invention
The present invention relates to a method for defoaming a vinyl chloride resin slurry. More specifically, the present invention relates to a method for defoaming a vinyl chloride resin slurry for preventing generation of Fish eyes and deterioration in production efficiency caused by slurry foams without inhibiting inherent transparency of vinyl chloride resins, in a case in which a reflux condenser or a heat removal apparatus such as a monomer vent is used in order to remove reaction heat, or in a case in which a specific compound is added in order to prevent slurry foams generated during serious gasification of monomers, for example, a stripping process, to remove unreacted monomers left behind after polymerization, in the process of preparing the vinyl chloride resins by polymerizing a monomer mixture containing vinyl chloride as a main component in a polymerization reactor.
2. Description of the Related Art
Recently, a method for preparing vinyl chloride resins to improve preparation efficiency, in which the size of a reactor is increased, a great amount of initiator and monomers are added at a constant volume, and a reflux condenser or liquid monomers is gasified to remove a great amount of reaction heat generated during the addition, thereby controlling the reaction using the gasification heat, is widely used.
Furthermore, a method in which hot polymerization water is directly added to a reactor in order to shorten a polymerization time by rapidly heating initially added reactants to a polymerization temperature, so-called “hot charge” is used.
A dispersant having a high saponification value is widely used as a dispersant for stabilization of monomer droplets in order to implement such polymerization. There is a problem in which the surface of vinyl chloride resins prepared using the dispersant having a saponification value has higher hydrophilicity and is readily bonded to moisture, to form slurry foams consisting of slurry and water, and the slurry foams then float to the top of the reactor.
The vinyl chloride resin that reaches the top of the reactor in the form of a mixture with water is adhered to the top of the reactor or the top of the tube of the reflux condenser which is difficult to wash. Such resin particles are not discharged and remain when not washed with water, and then participate in subsequent polymerization together with monomers. In this case, the produced re-polymerized particles have considerably low plasticizer absorbance and a possibility of having a large particle size, and are not readily melted during processing of the resins. As a result, Fish eyes are formed.
Furthermore, the foams that reach the outer wall and reflux condenser of the reactor significantly deteriorate heat transfer efficiency and decrease a normal production efficiency of the reactor to 30% or more. For this reason, it is important to prevent elevation of slurry foams during reaction in order to maintain a high preparation efficiency and a low Fish eye content of final products.
Well-known conventional methods include use of an anti-foaming agent selected from one of silicone and alkylene glycol antifoaming agents and amide anti-foaming agents, and an anti-foaming agent selected from polyvinyl acetate and cellulose derivatives disclosed in Japanese Patent Publication No. 2002-256018, use of an emulsion of fatty acid glyceride and polyalkylene glycol disclosed in Japanese Patent Publication No. 2002-348307, and use of a polyether aqueous solution disclosed in Japanese Patent Publication No. 2004-238522 or Japanese Patent Publication No. 2005-281659.
The techniques developed to date use continuous addition of an additive having a low interfacial tension to water or an emulsion of the additive in a dispersant during reaction in common. These additives function to inhibit slurry foaming, but disadvantageously deteriorate transparency due to lack of miscibility (solubility) of used anti-foaming agent in a vinyl chloride resin. In particular, antifoaming agents continuously added during reaction are used in a large amount of about 200 ppm, thus having a negative effect on transparency and damaging printability.
Accordingly, there is increasing demand for research on techniques to prevent generation of Fish eyes and deterioration in production efficiency caused by slurry foams without inhibiting inherent transparency of a vinyl chloride resin.